A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as cementing. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed.
One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas. Squeeze cementing is a remedial operation used to accomplish different objectives. For example squeeze cementing may be carried out in order to isolate a producing zone from zones which produce unwanted fluids; to seal of thief zones or loss circulation zones; to seal off perforations in zones which have been depleted; to fix corroded casing leaks; to prevent fluid migration in abandoned zones and wells; and/or to correct defects resulting from primary cementing jobs such as microannuli formation or the presence of flow channels. The latter four situations are typically encountered in zones which have casing and cement behind the casing. Squeeze cementing may be carried out in order to mitigate or correct for materials lost due to the presence of perforations that extend through the casing and cement, or to prevent the flow of unwanted fluids such as water into the well bore subsequent to depletion of the oil in the perforated and fractured zone. Casing leaks due to holes caused by corrosion may also allow unwanted formation fluids to be co-produced with the desired fluid, or allow for interzonal fluid communication. Additional adverse conditions that may be ameliorated by squeeze cementing include for example sustained casing pressure build up. Sustained casing pressure build up at the well head in producing or abandoned wells may be attributable to a number of factors such as fluid migration behind a casing through channels in cement column, or the presence of microannuli between the casing and cement, or between the cement and the formation. Sustained casing pressure build up can pose a number of hazards. Further, such casing pressure build ups can be economically disadvantageous as governmental agencies require the pressure build up not to exceed beyond some set values in abandoned wells. In producing wells, the sustained casing pressure build up may lead to casing and shoe failures. In injection and disposal wells, the presence of alternate flow paths in cased and cemented zones may lead to loss of injection pressures and fluid flow into undesired locations.
Alternate solutions to squeeze cementing include procedures to seal and plug the undesired fluid flow paths by injecting gelling fluids, pressure activated fluids and by mechanical isolations. All of these procedures including squeeze cementing are complex, laborious and frequently require multiple treatments to fix the problems. For example, the use of gelled fluids requires repeated experimentation to optimize the gel time which is dependent on a number of factors such as downhole temperatures and the ratio of components. Thus, there is a need for compositions which are simpler to use and formulate that can ameliorate the aforementioned adverse conditions.
Additionally, oil or gas residing in the subterranean formation may be recovered by driving the fluid into the well using, for example, a pressure gradient that exists between the formation and the wellbore, the force of gravity, or displacement of the fluid using a pump or the force of another fluid injected into the well or an adjacent well. The production of the fluid in the formation may be increased by hydraulically fracturing the formation. That is, a viscous fracturing fluid may be pumped down the casing to the formation at a rate and a pressure sufficient to form fractures that extend into the formation, providing additional pathways through which the oil or gas can flow to the well. Unfortunately, water rather than oil or gas may eventually be produced by the formation through the fractures therein, and such fluids may enter the wellbore through perforations in the production zone, or through high permeability channels in the case of a open hole production zone. To provide for the production of more oil or gas, a fracturing fluid may again be pumped into the formation to form additional fractures therein. However, the previously used fractures and associated fluid pathways first must be plugged to prevent the loss of the fracturing fluid into the formation via those fractures.
In addition to the fracturing fluid, other fluids used in servicing a wellbore may also be lost to the subterranean formation while circulating the fluids in the wellbore. For example, a drilling fluid may be lost to the formation, resulting in the circulation of the fluid in the wellbore being too low to allow for further drilling. Additionally, wellbore fluids used in injection wells to enhance hydrocarbon recovery or fluids designated for disposal may also be lost to the subterranean formation. Also, a secondary cement/sealant composition may be lost to the formation as it is being placed in the wellbore, thereby rendering the secondary operation ineffective in maintaining isolation of the formation.
In particular, the fluids may enter the subterranean formation via depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the drilling fluid, and so forth. As a result, the service provided by such fluids is more difficult to achieve. In some cases such fluids may enter natural aquifers that may supply drinking or agricultural water.
Accordingly, an ongoing need exists for compositions and methods of preventing the unwanted loss of fluids during wellbore servicing.